Recent experimental evidence suggests that the outer hair cells of the mammalian cochlea act as electromechanical amplifiers which increase hearing sensitivity one-hundred-fold. The long term goal of the proposed research is to confirm this hypothesis and to clarify our understanding of the underlying mechanisms. The specific aim of this proposal is to obtain experimental evidence in support of a detailed hypothesis describing the function of the cochlear amplifier. Our specific hypothesis is that the outer hair cell receptor current regulates a force-generation process which is located in the hair cell stereocilia. This force is coupled to the basilar and tectorial membranes and significantly increases the mechanical stimulus to the inner hair cells. We plan to stimulate the force-generation process through the injection of electrical current into scala media of the cochlea and to measure the resulting mechanical response as an otacoustic emission at the tympanic membrane. We will also use acoustic stimulation and measure the cochlear microphonic to study the role of the outer hair cell receptor current. We will study the effects of experimental manipulations which modify the amplification process. The effects of stimulation of the cochlear efferents on the force-generation process will also be studied. The results of the proposed experiments as well as previous experiments by ourselves and others will be interpreted with the aid of computational models. The computational models are physically based and include specific descriptions of the properties of the basilar and tectorial membranes as well as the hair cell transduction processes.